外文翻譯---關(guān)系數(shù)據(jù)庫(kù)的結(jié)構(gòu)-其他專業(yè)-全文預(yù)覽
【正文】 able, but it also prevents a row in the DEPT table from being deleted if there are employees still assigned to that department. Foreign Key:A column （ or columns） in a table that draws its values from a primary or unique key column in another table. A foreign key assists in ensuring the data integrity of a table. Referential IntegrityA method employed by a relational database system that enforces onetomany relationships between tables. Data Modeling Before Scott created the actual tables in the database, he went through a design process known as data modeling. In this process, the developer conceptualizes and documents all the tables for the database. One of the mon methods for modeling a database is called ERA, which stands for entities, relationships, and attributes. The database designer uses an application that can maintain entities, their attributes, and their relationships. In general, an entity corresponds to a table in the database, and the attributes of the entity correspond to columns of the table. Data Modeling:A process of defining the entities, attributes, and relationships between the entities in preparation for creating the physical database. The datamodeling process involves defining the entities, defining the relationships between those entities, and then defining the attributes for each of the entities. Once a cycle is plete, it is repeated as many times as necessary to ensure that the designer is capturing what is important enough to go into the database. Let39。s the DBA39。s widget pany database, he identifies four entities: employees, departments, salary grades, and bonuses. These will bee the EMP, DEPT, SALGRADE, and BONUS tables. Defining the Relationships Between Entities Once the entities are defined, the designer can proceed with defining how each of the entities is related. Often, the designer will pair each entity with every other entity and ask, Is there a relationship between these two entities? Some relationships are obvious。s assume for the moment that the majority of the database design is pleted and some tables need to be implemented. Scott creates the EMP table to hold the basic employee information, and it looks something like this: Notice that some fields in the Commission （ COMM） and Manager （ MGR） columns do not contain a value。ll be well on your way to leveraging the power of a relational database. Tables, Row, and Columns A table in a relational database, alternatively known as a relation, is a twodimensional structure used to hold related information. A database consists of one or more related tables. A row in a table is a collection or instance of one thing, such as one employee or one line item on an invoice. A column contains all the information of a single type, and the piece of data at the intersection of a row and a column, a field, is the smallest piece o
高考資料相關(guān)推薦
鄂ICP備17016276號(hào)-1